Support the latest HTTP Digest authentication specification (Issue #4862)

Support the latest HTTP Digest authentication specification (Issue #4862)

Also deprecates all httpMD5* functions.

- cgi-bin/var.c: Use cupsHashData to compute SID hash.
- cups/auth.c: Rewrite WWW-Authenticate parser to support multiple auth schemes
  and the new RFC 7616 version of HTTP Digest.
- cups/cups.h: Add cupsHashString function to get a hex version of a hash.
- cups/hash.c: Add MD5 support.
- cups/http.c: Track WWW-Authenticate in a long string, concatenate new set
  values.
- cups/http.h: Deprecate httpMD5* and recommend cupsDoAuth and cupsHash*.
- cups/http-private.h: Pull MD5 stuff, nonce_count is unsigned, track
  WWW-Authenticate header as a potentially long string.
- cups/http-support.c: Use cupsHashData to compute UUID hash.
- cups/md5.c: Comment everything out if we have an OS-supplied MD5 hash
  function.
- cups/md5passwd.c: Use cupsHash* functions.
- cups/tls-*.c: Use cupsHash* functions.
- cups/versioning.h: Add CUPS_API_2_3 definition.
- scheduler/client.c: Update WWW-Authenticate header to include AuthRef,
  Local, and PeerCred schemes with parameters as needed.